Engine combustion using gasoline fuel may generate particulate matter (PM) (such as soot and aerosols) that may be exhausted to the atmosphere. To enable emissions compliance, particulate filters (PF) may be included in the engine exhaust, to filter out exhaust PMs before releasing the exhaust to the atmosphere. Such devices may be periodically or opportunistically regenerated during operation of an engine to decrease the amount of trapped particulate matter. Regeneration is typically achieved by raising a temperature of the PF to a predetermined level for a sustained period, while flowing exhaust gas of a defined composition through the PF in order to burn or oxidize the trapped particulate matter.
Various approaches are provided for regenerating a PF in response to PM loading reaching a threshold amount. In one example, as shown in U.S. Pat. No. 8,833,060, Ruhland et al. disclose a method to increase exhaust temperature when regeneration conditions for a particulate filter are met. The exhaust temperature may be increased by retarding spark timing and/or by post-injection fueling. Further, heaters coupled to the exhaust passage upstream of the particulate filter may be used to increase the temperature of exhaust reaching the particulate filter.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, extended operation of the engine with a significant amount of spark retard from MBT for the purpose of exhaust heating may cause an increased variation in cylinder indicated mean effective pressure (IMEP) which may reduce combustion stability and trigger a misfire monitor.
The inventors herein have recognized that changing the compression ratio of an engine may have an effect on the uniformity of torque pulses causing engine vibrations as well as a temperature of the exhaust released by the cylinder. These effects may be leveraged for expediting heating of an exhaust particulate filter while reducing reliance on spark retard and improving engine smoothness. Thus in one example, the issues described above may be at least partly addressed by a method comprising: responsive to each of a higher than threshold load and a lower than threshold temperature at an exhaust particulate filter (PF), selectively lowering an engine compression ratio (CR), mechanically, via a variable compression ratio (VCR) mechanism, and selectively adjusting spark timing based on each of a PF temperature and an estimated residual gas fraction (RGF) at the lower CR. In this way, by first increasing exhaust temperature via engine compression ratio adjustments, and then adjusting spark timing based on residual gas fraction, exhaust temperature may be increased for opportunistic regeneration of a PF with decreased engine roughness.
As one example, once a PM load of an exhaust PF reaches a threshold load, the PF temperature may be increased to above a threshold temperature to burn the accumulated load. During operation with a torque converter in a locked position, in order to increase the exhaust temperature (and consequently the PF temperature) to the threshold temperature without increasing engine noise, vibration, and harshness (NVH), the compression ratio (CR) of the engine may be lowered via actuation of a variable compression device, for example to a lowest possible compression ratio. Operating the engine at the lower compression ratio may reduce the engine efficiency relative to the higher compression ratio while increasing the engine out temperature and producing uniform torque pulses. In addition to increasing the exhaust temperature, engine operation at the lower CR may result in a higher amount of residual gas remaining in the engine cylinders, increasing the residual gas fraction (RGF). An increase in the RGF may slow down the combustion process, thereby shifting the spark timing stability limit towards the maximum brake torque (MBT) timing. If after lowering the compression ratio to the lower limit, the PF temperature remains below the threshold temperature, a further increase the PF temperature may be achieved by retarding spark while staying in advance of the spark timing stability limit at the lower CR. During operation with a torque converter in an unlocked position, a higher amount of spark retard may be applied while operating the engine at a higher CR. Once the PF temperature reaches the threshold temperature, the PF may be regenerated opportunistically, or by actively enleaning the engine, and the regeneration history may be updated.
In this way, by lowering an engine CR to increase the exhaust temperature, the reliance on spark retard is reduced and PF regeneration may be enabled with decreased noise, vibration, and harshness (NVH). By adjusting spark timing at the lower CR based on an updated spark timing stability limit, a smaller amount of spark retard from MBT may be applied to attain the desired PF regeneration temperature. By retarding spark timing to within the spark timing stability limit, as updated based on RGF at the lower CR, combustion stability may be maintained and possibility of knock and misfire may be decreased. The technical effect of adjusting the engine CR based on a torque converter position is that engine smoothness during engine operation with spark timing retarded from MBT may be improved. By leveraging the effect of a reduction in compression ratio on exhaust residual amount and temperature, PF regeneration may be expedited, improving engine performance, emissions quality, and fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.